list assertion for python_requirement_library's requirements

Testing Done:
Ran new tests.

Reviewed at https://rbcommons.com/s/twitter/r/1142/
There are cases wehre we access a field on a payload that doesn't exist.

Rather than try to figure out when target.payload.excludes exists,
I added a method Payload.get_field_value() that directly access the
value and has the capability to return 'None' for a missing field.